The invention relates to a material wherein the main constituent is PVC, for manufacturing pipes to be used for relining underground pipelines by the pipe in folded condition being drawn into the pipeline and being engaged with the inside surface of the pipeline by temperature increase.
Several methods exist for restoring damaged or untight pipelines. Methods which provide engagement between the inserted relining pipe and the existing pipeline are of special interest because the capacity of the pipeline then will be reduced slightly or will not be reduced at all. In such a method a helically wound pipe is drawn into the pipeline and is brought to expand (e.g. Rib-Loc). Other methods are based on so called stockings comprising protecting layers and curable resin therebetween (e.g. in situ form), or pipes, usually of polyethylene (PE) wherein the cross section is reduced, with or without heating, by drawing the pipe through roller tools (e.g. Swage-lining).
In another prior art method a thermoplastic pipe which has been folded in connection with the extrusion thereof is drawn into an existing pipeline and is brought to engage the existing pipeline wall by inner over-pressure and temperature increase, so called coating.
There are today mainly two types of relining system based on folded pipes. One type includes systems with pipes manufactured of polyethylene (PE) or crosslinked ethylene plastic (PEX). These materials are relatively flexible at room temperature and, therefore, can be wound onto and unwound from a roll in a folded condition without heating. Storing and shipping of pipes on a roll is desired because joining of folded pipes by welding is complicated. Preferably the roll should have an inner diameter &lt;1.5 m in order to facilitate shipping of long pipes or pipes of large diameter. However, said materials lose their rigidity relatively slowly when heated. This means that the pipes have to be heated to 115.degree.-130.degree. C. in order to resume the original shape thereof, which means in practice that steam under relatively high pressure must be used, which in turn necessitates the use of complicated and expensive equipment. Since polyethylene moreover cannot be heated to a temperature above 130.degree. C. without unacceptable flow arising in the material there is obtained a very narrow "process window". Particularly when the pipelines are long or when they are located below the ground water level this provides a great drawback.
Another system is based on pipes made of polyethylene chloride (PVC). At room temperature PVC is relatively rigid and therefore must be heated before winding on a roll as well as unwinding therefrom in folded condition. Moreover, continuous heating of the pipe is required during the procedure of drawing the pipe into the pipeline. This is a serious practical drawback which requires extensive arrangements. On the contrary it is sufficient to heat pipes of PVC to 75.degree.-90.degree. C. in order that such pipes will return to their original cylindrical shape.
SE-B-454 536 discloses an inner lining for protection of the internal surface of a pipe. The lining is extruded as a tube, and is then reduced in size by deformation at a temperature below the recrystallisation temperature of the tube material so that it can be readily installed. After installation, the tube is then heated so that it resumes its original shape, to fit the inside of the pipe. Such heating can be effected by a heating source such as hot air being introduced to impart to the tube the required shape.
EP-A-0 353 525 discloses a thermoplastic elastomer composition of a PVC resin to which nitrite butyl rubber (NBR) is added in order to reduce the compression set of the thermoplastic elastomer, i.e. the remaining deformation after compression. The composition used is heated to a temperature of 150.degree.-210.degree. C. in order to achieve thermoplasticity.